Apparatus and methods for envelope tracking calibration

ABSTRACT

Apparatus and methods for envelope tracking calibration are provided. In one embodiment, a method of calibrating an envelope tracker having an envelope shaping table generated at a desired gain compression of a power amplifier is provided. The method includes generating a supply voltage for the power amplifier using the envelope tracker, operating the supply voltage of the power amplifier at a first voltage level associated with substantially no gain compression of the power amplifier, and measuring an output power of the power amplifier at the first voltage level. The method further includes decreasing a voltage level of the supply voltage one or more times and measuring the output power at each voltage level, determining a second voltage level of the power amplifier associated with a gain compression equal to about that of the desired gain compression, and calibrating the envelope tracker based on the determination.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e)of U.S. Provisional Patent Application No. 61/440,291, filed Feb. 7,2011 entitled “APPARATUS AND METHODS FOR ENVELOPE TRACKING CALIBRATION”,which is herein incorporated by reference in its entirety.

BACKGROUND

1. Field

Embodiments of the invention relate to electronic systems, and inparticular, to radio frequency (RF) electronics.

2. Description of the Related Technology

Power amplifiers can be included in mobile phones to amplify a RF signalfor transmission. For example, in mobile phones having a time divisionmultiple access (TDMA) architecture, such as those found in GlobalSystem for Mobile Communications (GSM), code division multiple access(CDMA), and wideband code division multiple access (W-CDMA) systems, apower amplifier can be used to amplify a RF signal for transmission viaan antenna. It can be important manage the amplification of a RF signal,as a desired transmitted power level can depend on how far the user isaway from a base station and/or the mobile environment. Power amplifierscan also be employed to aid in regulating the power level of the RFsignal over time, so as to prevent signal interference from transmissionduring an assigned receive time slot.

The power efficiency of a power amplifier at a particular input powerlevel can be a function of a variety of factors, including circuitcomponent and layout, power amplifier load, and/or power amplifiersupply voltage. To aid in improving the efficiency of a power amplifier,a technique known as envelope tracking can be used, in which the voltagelevel of the power supply of the power amplifier is changed in relationto the envelope of the RF signal. Thus, when the envelope of the RFsignal increases, the voltage supplied to the power amplifier can beincreased. Likewise, when the envelope of the RF signal decreases, thevoltage supplied to the power amplifier can be decreased to reduce powerconsumption.

SUMMARY

In certain embodiments, the present disclosure relates to a method ofcalibrating an envelope tracking system. The method includes generatinga supply voltage for a power amplifier using an envelope tracker, theenvelope tracker having an envelope shaping table generated at a desiredgain compression of the power amplifier. The method further includesoperating the supply voltage of the power amplifier at a first voltagelevel, the first voltage level associated with substantially no gaincompression of the power amplifier. The method further includesmeasuring an output power of the power amplifier at the first voltagelevel, decreasing a voltage level of the supply voltage one or moretimes and measuring the output power at each voltage level, determininga second voltage level of the power amplifier associated with a gaincompression equal to about that of the desired gain compression, andcalibrating the envelope tracker based on the determination.

In various embodiments, the method further includes scaling an amplitudeof an envelope signal to generate a scaled envelope signal, the supplyvoltage generated at least in part based on the scaled envelope signal.

In some embodiments, the envelope shaping table includes shaping datarelating a plurality of scaled envelope signal amplitudes to a pluralityof supply voltage levels.

In a number of embodiments, the method further includes generating thesupply voltage from a battery voltage using the shaping data and thescaled envelope signal.

In accordance with several embodiments, the shaping data is in a digitalformat, and the method further includes converting the shaping data intoan analog format.

In certain embodiments, decreasing the voltage level of the supplyvoltage includes changing calibration data of the envelope tracker toreduce the supply voltage.

In some embodiments, calibrating the envelope tracker based on thedetermination includes selecting a value of the calibration data equalto about a value of the calibration data corresponding to the secondvoltage level.

According to a number of embodiments, scaling the amplitude of theenvelope signal includes multiplying the envelope signal by a scalingfactor determined at least in part by the calibration data.

In various embodiments, the scaling factor is further determined by apower control signal from a transceiver.

In some embodiments, measuring the output power of the power amplifierat the first voltage level includes measuring the output power using adirectional coupler and a power detector electrically coupled to theoutput of the power amplifier.

In a number of embodiments, the first voltage level is equal to about amaximum supply voltage of the power amplifier.

In accordance with several embodiments, decreasing the voltage level ofthe supply voltage one or more times includes decreasing the voltagelevel in discrete steps.

In various embodiments, decreasing the voltage level of the supplyvoltage one or more times and measuring the output power at each voltagelevel includes decreasing the voltage level continuously and measuringthe output power at a plurality of discrete voltage levels.

In certain embodiments, the present disclosure relates to acomputer-readable storage medium including instructions that whenexecuted by a processor perform a method of calibrating an envelopetracking system. The method includes generating a supply voltage for apower amplifier using an envelope tracker, the envelope tracker havingan envelope shaping table generated at a desired gain compression of thepower amplifier. The method further includes operating the supplyvoltage of the power amplifier at a first voltage level, the firstvoltage level associated with substantially no gain compression of thepower amplifier. The method further includes measuring an output powerof the power amplifier at the first voltage level, decreasing a voltagelevel of the supply voltage one or more times and measuring the outputpower at each voltage level, determining a second voltage level of thepower amplifier associated with a gain compression equal to about thatof the desired gain compression, and calibrating the envelope trackerbased on the determination.

In certain embodiments, the present disclosure relates to a poweramplifier system including a power amplifier and an envelope trackerconfigured to generate a supply voltage for the power amplifier. Theenvelope tracker includes a shaping module having an envelope shapingtable generated at a desired gain compression of the power amplifier anda scaling module configured to scale an amplitude of an envelope signaland to provide the scaled envelope signal amplitude to the shapingmodule. The power amplifier system further includes a directionalcoupler electrically connected to an output of the power amplifier, apower detector electrically connected to the directional coupler andconfigured to measure the output power of the power amplifier using thedirectional coupler, and a calibration module configured to providecalibration data to the scaling module so as to change the scaledenvelope signal amplitude generated by the scaling module. Thecalibration module is configured to set the calibration data to a firstvalue corresponding to a voltage level of the supply voltage associatedwith substantially no gain compression, and to reduce the voltage levelof the supply voltage by changing the calibration data until the powerdetector indicates that the gain compression of the power amplifier isequal to about the desired gain compression.

In various embodiments, the envelope shaping table includes shaping datarelating a plurality of scaled envelope signal amplitudes to a pluralityof supply voltage levels.

In some embodiments, the power amplifier system further includes amodulator configured to generate the supply voltage from a batteryvoltage using the shaping data.

In a number of embodiments, the power amplifier system further includesa digital-to-analog converter for converting the shaping data to analogdata for use by the modulator.

In accordance with several embodiments, the power amplifier systemfurther includes a power control module electrically connected to thepower detector.

In certain embodiments, the scaling module is further configured toreceive a power control signal from the power control module and tochange the scaled envelope signal amplitude using the power controlsignal.

In some embodiments, the scaling module is configured to multiply thecalibration data by the power control signal to generate a scalingfactor, and to multiply the amplitude of the envelope signal by thescaling factor to generate the scaled envelope signal amplitude.

In various embodiments, the first value of the calibration datacorresponds to about a maximum supply voltage of the power amplifier.

In some embodiments, the power amplifier system further includes aduplexer having an input electrically connected to an output of thepower amplifier and to the directional coupler and an outputelectrically connected to an antenna.

In certain embodiments, the present disclosure relates to a method ofcalibrating a power amplifier system. The method includes generating asupply voltage for a power amplifier using an envelope tracker, theenvelope tracker having an envelope shaping table generated at a desiredgain compression of the power amplifier. The method further includesoperating the supply voltage of the power amplifier at a first voltagelevel and a first input power level associated with a target power ofthe power amplifier. The method further includes measuring an outputpower of the power amplifier at the first input power level to determinea power gain, increasing the input power of the power amplifier one ormore times and measuring the output power at each input power level,determining a second input power level corresponding to a gaincompression of the power amplifier equal to about the desired gaincompression, and calibrating the power amplifier system based on thedetermination.

In various embodiments, the method further includes using calibrationdata to change a gain of a variable gain amplifier configured to drivean input of the power amplifier.

In some embodiments, increasing the input power of the power amplifierone or more times includes changing the calibration data so as toincrease a gain of the variable gain amplifier one or more times.

In a number of embodiments, calibrating the power amplifier system basedon the determination includes selecting a value of the calibration dataequal to about a value of the calibration data corresponding to thesecond input power level.

In certain embodiments, the method further includes using a powercontrol signal from a transceiver to further control the gain of thevariable gain amplifier.

According to a number of embodiments, the method further includescontrolling the gain of the variable gain amplifier by multiplying thecalibration data by the power control signal.

In several embodiments, measuring the output power of the poweramplifier at the first input power level to determine the power gainincludes measuring the output power using a directional coupler and apower detector electrically coupled to the output of the poweramplifier.

In certain embodiments, the present disclosure relates to acomputer-readable storage medium including instructions that whenexecuted by a processor perform a method of calibrating a poweramplifier system. The method includes generating a supply voltage for apower amplifier using an envelope tracker, the envelope tracker havingan envelope shaping table generated at a desired gain compression of thepower amplifier. The method further includes operating the supplyvoltage of the power amplifier at a first voltage level and a firstinput power level associated with a target power of the power amplifier.The method further includes measuring an output power of the poweramplifier at the first input power level to determine a power gain,increasing the input power of the power amplifier one or more times andmeasuring the output power at each input power level, determining asecond input power level corresponding to a gain compression of thepower amplifier equal to about the desired gain compression, andcalibrating the power amplifier system based on the determination.

In certain embodiments, the present disclosure relates to a poweramplifier system including a power amplifier, a variable gain amplifierconfigured to drive an input of the power amplifier, and an envelopetracker configured to generate a supply voltage for the power amplifier.The envelope tracker includes an envelope shaping table generated at adesired gain compression of the power amplifier. The power amplifiersystem further includes a directional coupler electrically connected toan output of the power amplifier, a power detector electricallyconnected to the directional coupler and configured to measure theoutput power of the power amplifier using the directional coupler, and acalibration module configured to provide calibration data to thevariable gain amplifier so as to control the input power of the poweramplifier. The calibration module is configured to set the calibrationdata to a first value corresponding to a voltage level of the supplyvoltage and input power of the power amplifier associated with a targetpower of the power amplifier, and to increase the input power of thepower amplifier by changing the calibration data until the powerdetector indicates that the gain compression of the power amplifier isequal to about the desired gain compression.

In various embodiments, the power amplifier system further includes apower control module electrically connected to the power detector.

In some embodiments, the power control module is configured to generatea power control signal for controlling the gain of the variable gainamplifier.

In a number of embodiments, the power amplifier system includes amultiplier for multiplying the calibration data by the power controlsignal to generate a gain control signal for controlling the gain of thevariable gain amplifier.

In accordance with certain embodiments, the power amplifier systemfurther includes a duplexer having an input electrically connected to anoutput of the power amplifier and to the directional coupler and anoutput electrically connected to an antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an example wireless device thatcan include one or more power amplifier modules.

FIG. 2 is a schematic block diagram of one example of a power amplifiersystem having an envelope tracker.

FIGS. 3A-3B show two examples of a power supply voltage versus time.

FIG. 4 is a schematic block diagram of another example of a poweramplifier system having an envelope tracker.

FIG. 5 is a plot illustrating one example of supply voltage and gainversus input power.

FIG. 6 is a schematic block diagram of a power amplifier system inaccordance with one embodiment.

FIG. 7 is a schematic block diagram of a power amplifier system inaccordance with another embodiment.

FIG. 8 is a flow chart illustrating a method of calibrating a poweramplifier system in accordance with one embodiment.

FIG. 9 is a flow chart illustrating a method of calibrating a poweramplifier system in accordance with another embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

The headings provided herein, if any, are for convenience only and donot necessarily affect the scope or meaning of the claimed invention.

It can be difficult to maintain certain performance characteristics of apower amplifier when varying the power amplifier supply voltage inrelation to the envelope of the RF signal. For example, part-to-partcomponent variation within the system can produce a misalignment betweenan envelope voltage and an associated power supply voltage generated bythe envelope tracker, thereby making it difficult to maintain arelatively constant gain compression when tracking an envelope signalover a wide dynamic range. Although a power amplifier can be calibratedto try to compensate for the error, the calibration can be complicatedby variations in DC offset voltage, insertion loss and/or gain in theenvelope and signal paths.

There is a need for improved power amplifiers. Furthermore, there is aneed for improved apparatus and methods for envelope trackingcalibration.

FIG. 1 is a schematic block diagram of an example wireless device 11that can include one or more power amplifier modules. The wirelessdevice 11 can include power amplifiers implementing one or more featuresof the present disclosure.

The example wireless device 11 depicted in FIG. 1 can represent amulti-band and/or multi-mode device such as a multi-band/multi-modemobile phone. By way of examples, Global System for Mobile (GSM)communication standard is a mode of digital cellular communication thatis utilized in many parts of the world. GSM mode mobile phones canoperate at one or more of four frequency bands: 850 MHz (approximately824-849 MHz for Tx, 869-894 MHz for Rx), 900 MHz (approximately 880-915MHz for Tx, 925-960 MHz for Rx), 1800 MHz (approximately 1710-1785 MHzfor Tx, 1805-1880 MHz for Rx), and 1900 MHz (approximately 1850-1910 MHzfor Tx, 1930-1990 MHz for Rx). Variations and/or regional/nationalimplementations of the GSM bands are also utilized in different parts ofthe world.

Code division multiple access (CDMA) is another standard that can beimplemented in mobile phone devices. In certain implementations, CDMAdevices can operate in one or more of 800 MHz, 900 MHz, 1800 MHz and1900 MHz bands, while certain WCDMA and Long Term Evolution (LTE)devices can operate over, for example, about 22 radio frequency spectrumbands.

One or more features of the present disclosure can be implemented in theforegoing example modes and/or bands, and in other communicationstandards. For example, 3G, 4G, LTE and Advanced LTE are non-limitingexamples of such standards.

In certain embodiments, the wireless device 11 can include a duplexer12, a transceiver 13, an antenna 14, power amplifiers 17, a controlcomponent 18, a computer readable medium 19, a processor 20, a battery21, and an envelope tracker 22.

The transceiver 13 can generate RF signals for transmission via theantenna 14. Furthermore, the transceiver 13 can receive incoming RFsignals from the antenna 14.

It will be understood that various functionalities associated with thetransmission and receiving of RF signals can be achieved by one or morecomponents that are collectively represented in FIG. 1 as thetransceiver 13. For example, transmitting and receiving functionalitiescan be provided by separate components.

Similarly, it will be understood that various antenna functionalitiesassociated with the transmission and receiving of RF signals can beachieved by one or more components that are collectively represented inFIG. 1 as the antenna 14. For example, a single antenna can beconfigured to provide both transmitting and receiving functionalities.In another example, transmitting and receiving functionalities can beprovided by separate antennas. In yet another example, different bandsassociated with the wireless device 11 can be provided with one or moreantennas.

In FIG. 1, one or more output signals from the transceiver 13 aredepicted as being provided to the antenna 14 via one or moretransmission paths 15. In the example shown, different transmissionpaths 15 can represent output paths associated with different bandsand/or different power outputs. For example, two example poweramplifiers 17 shown can represent amplifications associated withdifferent power output configurations (e.g., low power output and highpower output), and/or amplifications associated with different bands.Although FIG. 1 illustrates the wireless device 11 as including twotransmission paths 15, the wireless device 11 can be adapted to includemore or fewer transmission paths 15.

In FIG. 1, one or more detected signals from the antenna 14 are depictedas being provided to the transceiver 13 via one or more receiving paths16. In the example shown, different receiving paths 16 can representpaths associated with different bands. For example, the four examplepaths 16 shown can represent quad-band capability that some wirelessdevices are provided with. Although FIG. 1 illustrates the wirelessdevice 11 as including four receiving paths 16, the wireless device 11can be adapted to include more or fewer receiving paths 16.

To facilitate switching between receive and transmit paths, the duplexer12 can be configured to electrically connect the antenna 14 to aselected transmit or receive path. Thus, the duplexer 12 can provide anumber of switching functionalities associated with an operation of thewireless device 11. In certain embodiments, the duplexer 12 can includea number of switches configured to provide functionalities associatedwith, for example, switching between different bands, switching betweendifferent power modes, switching between transmission and receivingmodes, or some combination thereof. The duplexer 12 can also beconfigured to provide additional functionality, including filtering ofsignals.

FIG. 1 shows that in certain embodiments, a control component 18 can beprovided, and such a component can be configured to provide variouscontrol functionalities associated with operations of the duplexer 12,the power amplifiers 17, the envelope tracker 22, and/or other operatingcomponent(s). Non-limiting examples of the control component 18 aredescribed herein in greater detail.

In certain embodiments, a processor 20 can be configured to facilitateimplementation of various processes described herein. For the purpose ofdescription, embodiments of the present disclosure may also be describedwith reference to flowchart illustrations and/or block diagrams ofmethods, apparatus (systems) and computer program products. It will beunderstood that each block of the flowchart illustrations and/or blockdiagrams, and combinations of blocks in the flowchart illustrationsand/or block diagrams, may be implemented by computer programinstructions. These computer program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the acts specified in the flowchart and/or block diagramblock or blocks.

In certain embodiments, these computer program instructions may also bestored in a computer-readable memory 19 that can direct a computer orother programmable data processing apparatus to operate in a particularmanner, such that the instructions stored in the computer-readablememory produce an article of manufacture including instruction meanswhich implement the acts specified in the flowchart and/or block diagramblock or blocks. The computer program instructions may also be loadedonto a computer or other programmable data processing apparatus to causea series of operations to be performed on the computer or otherprogrammable apparatus to produce a computer implemented process suchthat the instructions that execute on the computer or other programmableapparatus provide steps for implementing the acts specified in theflowchart and/or block diagram block or blocks.

The illustrated wireless device 11 also includes the envelope tracker22, which can be used to provide a power supply voltage to one or moreof the power amplifiers 17. For example, the envelope tracker 22 can beconfigured to control the supply voltage provided to the poweramplifiers 17 based upon an envelope of the RF signal to be amplified.

The envelope tracker 22 can be electrically connected to a battery 21,and the envelope tracker 22 can be configured to vary or change thevoltage provided to the power amplifiers 17 based on an envelope of theRF signal to be amplified. The battery 21 can be any suitable batteryfor use in the wireless device 11, including, for example, a lithium-ionbattery. As will be described in detail further below, by controllingthe voltage provided to the power amplifiers, the power consumption ofthe battery 21 can be reduced, thereby improving performance of thewireless device 11. The envelope signal can be provided to the envelopetracker 22 from the transceiver 13. However, the envelope signal can bedetermined in other ways. For example, the envelope signal can bedetermined by detecting the envelope of the RF signal using any suitableenvelope detector.

FIG. 2 is a schematic block diagram of one example of a power amplifiersystem 50 having an envelope tracker. The illustrated power amplifiersystem 50 includes the battery 21, a power amplifier 32, a multi-levelsupply module 51, a supply voltage selection module 52, and a supplyvoltage adjustment module 54. As will be described below, themulti-level supply module 51, the supply voltage selection module 52,and the supply voltage adjustment module 54 can collectively operate asan envelope tracker configured to vary or change a power supply voltageV_(CC) in relation to the envelope signal.

The power amplifier 32 includes an input configured to receive an RFsignal RF_(IN) and an output configured to generate an amplified RFsignal RF_(OUT). Additionally, the power amplifier 32 is electricallypowered using the power supply voltage V_(CC).

The multi-level supply module 51 can generate a plurality of powersupplies from the battery 21. For example, the multi-level supply module51 can be used to generate n supplies from the battery 21, where n is aninteger. Each of the supplies generated by the multi-level supply module51 can have a voltage level that is greater than, less than, or equal tothe battery voltage. In one implementation, the multi-level supplymodule 51 includes a buck-boost converter.

The supply voltage selection module 52 can receive the envelope of theRF signal RF_(IN), and can select amongst the supplies generated by themulti-level supply module 51 to provide the supply voltage adjustmentmodule 54 with a supply voltage level that is most appropriate to use totrack the envelope signal. For example, the supply voltage selectionmodule 52 can provide the supply voltage adjustment module 54 a supplyvoltage that is greater than the envelope voltage by a relatively smallamount. Thereafter, the supply voltage adjustment module 54 can providerelatively fine-tuned adjustment of the supply voltage to generate theenvelope tracking power supply voltage V_(CC). By including themulti-level supply module 51, the supply voltage selection module 52,and the voltage adjustment module 54, constraints on the design of theenvelope tracking system can be reduced, thereby permitting a systemwith greater flexibility and improved power efficiency relative to ascheme employing only a single tracking or selection module.

As illustrated in FIG. 2, the supply voltage adjustment module 54 can beelectrically connected in a feedback arrangement to aid in enhancing thetracking of the power supply V_(CC) relative to the envelope of the RFsignal. The supply voltage adjustment module 54 can include one or moreamplifiers configured to provide linear tracking of the envelope signalto generate the power supply voltage V_(CC). In certain implementations,one or more amplifiers can be electrically connected with one or moresummers to aid in aid in generating an error signal, which can be addedto the supply voltage selected by the supply voltage selection module52. Although FIG. 2 illustrates a feedback configuration in which thepower supply voltage V_(CC) is provided as an input back into the supplyvoltage adjustment module 54, in certain implementations, a feedforwardarrangement can be used.

Although not illustrated in FIG. 2, the power amplifier system 50 caninclude a delay block to compensate for delays in generating the powersupply voltage V_(CC). For example, a delay block can be includedbetween the RF signal RF_(IN) and the input of the power amplifier 32 toaid in aligning the signal amplified by the power amplifier 32 with thepower supply voltage V_(CC).

FIGS. 3A-3B show two examples of power supply voltage versus time.

In FIG. 3A, a graph 47 illustrates the voltage of an RF signal 41 and apower amplifier supply 43 versus time. The RF signal 41 has an envelope42.

The power supply 43 of a power amplifier can be configured to have avoltage greater than that of the RF signal 41. For example, providing asupply voltage to a power amplifier having a voltage magnitude less thanthat of the RF signal 41 can clip the RF signal, thereby creating signaldistortion and/or other problems. Thus, the power supply 43 can beselected to have a voltage magnitude that is greater than that of theenvelope 42 of the RF signal 41. However, it can be desirable to reducea difference in voltage between the power supply 43 and the envelope 42of the RF signal 41, as the area between the power amplifier supply 43and the envelope 42 of the RF signal 41 can represent lost energy, whichcan reduce battery life and increase heat generated in a mobile device.

In FIG. 3B, a graph 48 illustrates the voltage of an RF signal 41 and apower amplifier supply 44 versus time. In contrast to the poweramplifier supply 43 of FIG. 3A, the power amplifier supply 44 of FIG. 3Bchanges in relation to the envelope 42 of the RF signal 41. The areabetween the power amplifier supply 44 and the envelope 42 of the RFsignal 41 in FIG. 3B is less than the area between the power amplifiersupply 43 and the envelope 42 of the RF signal 41 in FIG. 3A, and thusthe graph 48 of FIG. 3B can be associated with a power amplifier systemhaving greater energy efficiency. FIG. 3B can represent the output ofone example of an envelope tracking system, such as the envelopetracking systems described herein.

FIG. 4 is a schematic block diagram of another example of a poweramplifier system 60 having an envelope tracker 22. The illustrated poweramplifier system 60 includes the envelope tracker 22, a power amplifier32, an inductor 62, a bypass capacitor 63, an impedance matching block64, the duplexer 12, and the antenna 14.

The power amplifier 32 can receive a RF signal RF_(IN) and generate anamplified RF signal RF_(OUT). The envelope tracker 22 can receive anenvelope of the RF signal RF_(IN) and can generate a power amplifiersupply voltage V_(CC) for the power amplifier 32 that tracks theenvelope signal.

The illustrated power amplifier 32 includes a bipolar transistor 61having an emitter, a base, and a collector. The emitter of the bipolartransistor 61 can be electrically connected to a first voltage supplyV₁, which can be, for example, a ground supply or node. Additionally,the RF signal RF_(IN) can be provided to the base of the bipolartransistor 61. The bipolar transistor 61 can amplify the RF signalRF_(IN) to generate the amplified RF signal RF_(OUT) at the collector.The bipolar transistor 61 can be any suitable device. In oneimplementation, the bipolar transistor 61 is a heterojunction bipolartransistor (HBT).

The power amplifier 32 can be configured to provide the amplified RFsignal RF_(OUT) to the duplexer 12. The impedance matching block 64 canbe used to aid in terminating the electrical connected between the poweramplifier 32 and the duplexer 12. For example, the impedance matchingblock 64 can be used to increase power transfer and/or reducereflections of the amplified RF signal RF_(OUT). In certainimplementations, the inductor 62 can be configured to operate as part ofthe impedance matching block 64.

The inductor 62 can be included to aid in biasing the power amplifier 32with the power amplifier supply voltage V_(CC) generated by the envelopetracker 22. The inductor 62 can include a first end electricallyconnected to the envelope tracker 22, and a second end electricallyconnected to the collector of the bipolar transistor 61. The bypasscapacitor 63 can have a first end electrically connected to the powersupply V_(CC) and a second end electrically connected to the firstvoltage supply V₁, and can perform a wide variety of functions. Forexample, including the bypass capacitor 63 can reduce noise of thesupply voltage V_(CC) and/or stabilize the output of the power amplifier32. Additionally, the bypass capacitor 63 can be used to provide an RFand/or AC ground for the inductor 62.

Although FIG. 4 illustrates one implementation of the power amplifier32, skilled artisans will appreciate that the teachings described hereincan be applied to a variety of power amplifier structures, including,for example, multi-stage power amplifier structures and power amplifiersemploying other transistor structures. For example, in someimplementations the bipolar transistor 61 can be omitted in favor ofemploying a field-effect transistor (FET), such as a silicon FET, agallium arsenide (GaAs) high electron mobility transistor (HEMT), or alaterally diffused metal oxide semiconductor (LDMOS) transistor.

FIG. 5 is a plot 70 illustrating one example of supply voltage and gainversus input power. The plot 70 includes a first curve 71 illustrating apower amplifier's power supply voltage in volts versus input power indBm. The plot 70 further includes a second curve 72 illustrating poweramplifier gain in dB versus power amplifier input power in dBm for thepower amplifier.

The first and second curves 71, 72 illustrate that the gain of a poweramplifier can be maintained relatively constant as input power increasesby increasing a power supply voltage level of a power amplifier inrelation to the input power. For example, the gain shown in the secondcurve 72 has been maintained relatively constant at a magnitude of about13.25 dB by increasing the power supply voltage from about 1 V to about6 V when increasing the input power level from about −15 dBm to about 22dBm.

When performing envelope tracking, a relatively constant gaincompression of the power amplifier can be maintained by controlling thedifference between the power supply voltage level and the input power ofthe envelope signal. To aid in controlling the amplitude of the powersupply voltage relative to the amplitude of the envelope signal, anenvelope tracker can include an envelope shaping table generated at atarget gain compression that includes data relating a plurality ofdesired voltage supply amplitudes to a plurality of envelope signalamplitudes.

When using an envelope shaping table, part-to-part variations ofcomponents within the power amplifier system can introduce variationthat can create a misalignment between the generated power supplyvoltage and the actual input power. For example, offset voltages, poweramplifier gain errors, and/or a variety of other factors can lead to amisalignment between the amplitude of the power supply and the inputpower of the envelope signal. It can be difficult to compensate forthese errors using conventional calibration techniques for a variety ofreasons. For example, the power amplifier can include a duplexerelectrically connected between the output of the power amplifier and theantenna, and variation in insertion loss of the duplexer can make itdifficult to correlate power measurements at the antenna to the powersupply voltage of the power amplifier. Thus, duplexer loss uncertaintyand/or other losses between the power amplifier output and the antennacan complicate calibration.

To accommodate for part-to-part variations and/or other contributingfactors to envelope amplitude misalignment, a power amplifier system canbe designed to include margin to account for the envelope amplitudemisalignment error. For example, gain compression can exist near amaximum output power level of an amplifier and can introduce distortionin the RF signal, and a power amplifier can be operated with anincreased power supply voltage to provide additional headroom againstdistortion. However, increasing the power supply voltage can reduce theefficiency of the power amplifier.

There is a need for improving the calibration of a power amplifiersystem so as to increase power amplifier efficiency and to avoiddistortion of an amplified RF signal due to envelope amplitudemisalignment. Furthermore, there is a need for a calibration schemesthat have a relatively fast calibration time so as to reduce calibrationcost in a factory setting and/or to permit the use of the calibrationmethod dynamically in a mobile device operating environment. Moreover,there is a need for improved calibration systems that can account forduplexer loss uncertainty and/or losses between the output of a poweramplifier and an antenna.

FIG. 6 is a schematic block diagram of a power amplifier system 98 inaccordance with one embodiment. The power amplifier system 98 includes aduplexer 12, a transceiver 13, an antenna 14, a battery 21, an envelopetracker 22, a power amplifier input stage or variable gain amplifier(VGA) 31, a power amplifier 32, and a directional coupler 88.

The illustrated envelope tracker 22 includes a calibration module 80, ascaling module 81, a shaping table module 82, a digital-to-analogconverter 83, a modulator 84, and a multiplier 87. The illustratedtransceiver 13 includes a power control module 85 and a power detector86. As will be described in detail below, the calibration module 80 ofthe envelope tracker 22 can be used to calibrate the envelope tracker 22to accommodate for envelope amplitude misalignment.

The illustrated transceiver 13 is configured to provide an envelopesignal to the envelope tracker 22 and an RF signal RF_(IN) to the VGA31. The transceiver 13 includes the power control module 85, which canbe used to adjust a power level of the power amplifier system 98. Forexample, the power control module 85 can provide a first power controlsignal or gain control level (GCL) to the VGA 31, which can be used tocontrol a gain of the VGA 31. Additionally, the power control module 85can provide a second power control signal or power control level (PCL)to the envelope tracker 22, which can be used to scale the amplitude ofthe envelope signal. The power control module 85 can be used to controlthe power level of the power amplifier system 98 over a variety of powermodes and/or other power settings and to compensate for various systemand/or operational parameters that can impact power performance.

To aid in enhancing the accuracy of the power control module 85, thetransceiver 13 can include the power detector 86. The power detector 86can be electrically coupled to a directional coupler 88 positioned atthe output of the power amplifier 32 so as to improve output powermeasurement accuracy. For example, the directional coupler 88 can bepositioned between the output of the power amplifier 32 and the input ofthe duplexer 12, thereby allowing the power detector 86 to generate apower measurement that does not include an insertion loss of theduplexer 12. However, in certain implementations, the directionalcoupler 88 need not be positioned directly at the output of the poweramplifier 32. For example, FIG. 6 has illustrated in dashed lines analternative location for the directional coupler 88 between the duplexer12 and the antenna 14.

The scaling module 81 can receive the power control level (PCL) from thepower control module 85, and can use the PCL to scale an amplitude ofthe envelope signal. The scaled or amplified envelope signal can beprovided to the shaping table module 82, which can include a shapingtable having shaping data relating a plurality of scaled envelope signalamplitudes to a plurality of target supply voltage levels. The shapingtable can be generated at a particular target gain compression, as wasdescribed above with reference to FIG. 5.

The shaping table module 82 can generate a signal including dataindicating a desired supply voltage level, and provide the signal to amodulator 84. In implementations in which the signal is a digitalformat, such as in configurations in which the signal corresponds to anentry from the shaping table, the digital-to-analog converter 83 can beused to convert the signal to an analog format. The modulator 84 can beelectrically connected to the battery 21, and can use the data relatingto the target supply voltage level from the shaping table module 82 togenerate the power supply voltage V_(CC) for the power amplifier 32.

The envelope tracker 22 includes the calibration module 80, whichincludes calibration data that can be set to a variety of values. Thecalibration data can be provided to the scaling module 81, which can usethe calibration data to scale the amplitude of the envelope signalbefore providing the scaled amplitude signal to the shaping table module82. For example, as illustrated in FIG. 6, the multiplier 87 can beconfigured to multiply the calibration data from the calibration module80 by the power control level (PCL) from the power control module 85 andby the envelope signal to generate a scaled envelope signal. However,the scaling module 81 can use the calibration data to scale the envelopesignal in any suitable way.

The calibration module 80 can use the calibration data to calibrate theenvelope tracker 22 to correct for envelope amplitude misalignment usinga multi-step calibration process. For example, the calibration module 80can begin by setting the calibration data to a value that scales theenvelope signal value by a relatively large amount, thereby directingthe shaping table module 82 to set the power supply voltage V_(CC) to arelatively high value, such as a maximum power supply voltage of thepower amplifier. The relatively high voltage of the power supply cancorrespond to substantially no gain compression of the power amplifier32.

When the power amplifier system is configured to have substantially nogain compression as was described above, the power detector 86 can beconfigured to measure the output power of the power amplifier 32.Thereafter, the calibration module 80 can change the state or value ofthe calibration data so as to direct the target supply voltage generatedby the shaping table module 82 downward. For example, the scaling module81 can use the calibration data to reduce the scaling factor, therebycausing the shaping table module 82 to decrease the target power supplyvoltage.

For each decrease of power supply voltage, the power detector 86 canmeasure the output power, and provide the output power measurement tothe power control module 85. Using the information, the calibrationmodule 80 or any other suitable module can determine when the value ofthe calibration data corresponds to a gain compression equal to aboutthat used to generate the shaping table of the shaping table module 82.For example, when the shaping table of the shaping table module 82 isgenerated at a 2 dB gain compression, the calibration module 80 candetermine when the output power measured by the power detector 86 isabout 2 dB less than the output power measured when the power amplifiersystem was configured in a state having substantially no gaincompression.

The calibration data associated with a power amplifier gain compressionequal to that used to generate the shaping table of the shaping tablemodule 82 can be stored in the power amplifier system, such as in amemory of the envelope tracker 22. The calibration data can be used tocompensate for an envelope amplitude misalignment of the power amplifiersystem.

The calibration scheme described above can be relatively low cost, takea relatively short amount of time, and can be used to account forenvelope amplitude misalignment errors from a variety of sources.Additionally, using the power detector 86 for power measurements avoidsa need of using external test equipment to support the calibration.

In certain implementations, the calibration module 80 can be used toperform a factory-level calibration of the envelope tracker 22 duringmanufacture. However, in other implementations, the calibration module80 can periodically calibrate the envelope tracker 22 during real-timeoperation, thereby accounting for dynamic errors from temperature orother environmental factors and/or operating conditions that can varythe envelope amplitude misalignment error dynamically over time. Thecalibration can be performed during any suitable time window, such ascertain time instances when the power amplifier system is nottransmitting a signal over the antenna 14.

FIG. 7 is a schematic block diagram of a power amplifier system 99 inaccordance with another embodiment. The power amplifier system 99includes a duplexer 12, a transceiver 13, an antenna 14, a battery 21,an envelope tracker 22, a VGA 31, a power amplifier 32, and adirectional coupler 88.

The illustrated envelope tracker 22 includes a scaling module 81, ashaping table module 82, a digital-to-analog converter 83, and amodulator 84. The power amplifier system of FIG. 7 can be similar to thepower amplifier system of FIG. 6 described above. However, in contrastto the power amplifier system 98 illustrated in FIG. 6, the poweramplifier system 99 illustrated in FIG. 7 includes a calibration module90 and a multiplier 91 for controlling a gain of the VGA 31.

The calibration module 90 includes calibration data that can be set to avariety of values. The calibration data can be used to select the gainof the VGA 31, thereby controlling the input power provided to the poweramplifier stage 32. The calibration module 32 can be used to correct anenvelope amplitude misalignment of the envelope tracker, as will bedescribed below.

The calibration module 90 can use the calibration data to calibrate theenvelope tracker 22 to correct for envelope amplitude misalignment usinga multi-step calibration process. For example, the calibration module 90can begin by setting the gain of the VGA 31 to a maximum power controllevel, while the envelope tracker 22 can be configured to generate apower supply voltage that is of a nominal value and consistent with atarget power of the power amplifier system.

When the power amplifier system is configured to have the target power,the calibration module 90 can change the state of the calibration dataso as to direct the gain of the VGA 31 upward, thereby increasing theinput power of the power amplifier 32.

For each increase of input power, the power detector 86 can measure theoutput power, and provide the measurement to the power control module85. Using the information, the calibration module 90 or any othersuitable component of the power amplifier system can determine when thevalue of the calibration data corresponds to a gain compression equal toabout that used to generate the shaping table of the shaping tablemodule 82. For example, when the gain of the power amplifier falls belowthat associated with the target power due to an increase in input power,the power amplifier system has exceeded the gain compression point ofthe power amplifier system.

The calibration data corresponding to when the power amplifier gaincompression is equal to about the gain compression used to generate theshaping table can be stored in the power amplifier system, such as in amemory of the envelope tracker 22 or of the transceiver 13. Thecalibration data can be used to compensate for an envelope amplitudemisalignment of the power amplifier system 99. To permit the powercontrol module 85 to also change the gain of the VGA 31, the multiplier91 can be included so that both the calibration module 90 and the powercontrol module 85 can control the gain of the VGA 31. However, incertain implementations, the multiplier 91 can be replaced with othercomponents, such as an adder, or omitted.

FIG. 8 is a flow chart illustrating a method 100 of calibrating a poweramplifier system in accordance with one embodiment. It will beunderstood that the methods discussed herein may include greater orfewer operations and the operations may be performed in any order, asnecessary. The method 100 can be used to calibrate, for example, thepower amplifier system 98 illustrated in FIG. 6.

The method 100 starts at a block 102. In an ensuing block 104, a supplyvoltage for a power amplifier is generated using an envelope trackerthat includes a shaping table generated at a desired gain compression.Gain compression in a power amplifier can refer to a reduction indifferential gain caused by overdriving the power amplifier beyond alinear region. Thus, a shaping table can be calibrated at a desired gaincompression that is determined to be an acceptable level of gaincompression for the design, and the envelope shaping table can mapenvelope signal amplitude to a power supply voltage level thatcorresponds to the desired gain compression. The shaping table caninclude shaping data relating a plurality of scaled envelope signalamplitudes to a plurality of target power supply voltage levels.

In an ensuing block 106, the supply voltage of a power amplifier isoperated at a first voltage level associated with substantially no gaincompression of the PA. For example, the power amplifier can be operatedat a maximum power supply voltage so as to provide maximum headroom forthe amplified signal and substantially no gain compression.

The method 100 of FIG. 8 continues at a block 108, in which the outputpower of the power amplifier is measured at the first voltage level. Forexample, a power detector can be used to measure the output power.Persons having ordinary skill in the art will appreciate that measuringoutput power can consist of measuring a current, voltage, and/or anotherparameter computationally related to power, and calculating powertherefrom.

In an ensuing block 110, a voltage level of the supply voltage can bedecreased one or more times, and the output power can be measured ateach voltage level. The voltage level can be decreased discretely, withpower measurements made after each decrease. However, in certainimplementations the voltage level can be decreased continuously, andmeasurements can be made at discrete points or continuously. The powermeasurements can be made using a power detector or any other suitablecomponent. In one embodiment, the supply voltage is decreased bychanging calibration data in a calibration module of the power amplifiersystem.

The method 100 continues at a block 112, in which a second voltage levelof the power supply is determined that corresponds to a gain compressionequal to about the gain compression used to generate the envelopeshaping table. For example, the voltage level can be decreased until themeasured output power falls below the output power at the first supplyvoltage by an amount equal to about the gain compression of the envelopeshaping table.

In an ensuing block 114, the envelope tracker is calibrated based on thedetermination. For example, a calibration data corresponding to thestate of the system at the second voltage level can be stored and usedto calibrate the power amplifier system. The method 100 ends at 116.

FIG. 9 is a flow chart illustrating a method of calibrating a poweramplifier system in accordance with another embodiment. It will beunderstood that the methods discussed herein may include greater orfewer operations and the operations may be performed in any order, asnecessary. The method 150 can be used to calibrate, for example, thepower amplifier system 99 illustrated in FIG. 7.

The method 150 starts at a block 152. In an ensuing block 154, a supplyvoltage for a power amplifier is generated using an envelope trackerincludes a shaping table generated at a desired gain compression point.The shaping table can include shaping data relating a plurality ofscaled envelope signal amplitudes to a plurality of desired supplyvoltage levels.

In an ensuing block 156, the supply voltage of a power amplifier isoperated at a first voltage level and a first input power levelassociated with the target power. For example, the power amplifier canbe operated at a supply voltage level below the maximum and at arelatively low input power consistent with the target power.

The method 150 of FIG. 9 continues at a block 158, in which the outputpower of the power amplifier is measured at the first voltage level andthe first input power level to determine a power gain. For example, apower detector can be used to measure the output power.

In an ensuing block 160, an input power of the power amplifier isincreased one or more times, and the output power can be measured ateach voltage level. The input power can be increased in any suitableway, such as by changing the gain of a variable gain amplifierconfigured to drive the input of a power amplifier.

The method 150 continues at a block 162, in which a second input powerlevel of the power supply is determined that corresponds to a gaincompression equal to about the gain compression used to generate theenvelope shaping table. For example, the input power can be decreaseduntil the gain begins to decrease, thereby indicating that the gaincompression has exceeded the gain compression used to determine theenvelope shaping table.

In an ensuing block 164, the envelope tracker is calibrated based on thedetermination. For example, a calibration data corresponding to thestate of the system at the second input power level can be stored andused to calibrate the power amplifier system. The method 150 ends at166.

Applications

Some of the embodiments described above have provided examples inconnection with mobile phones. However, the principles and advantages ofthe embodiments can be used for any other systems or apparatus that haveneeds for power amplifier systems.

Such power amplifier systems can be implemented in various electronicdevices. Examples of the electronic devices can include, but are notlimited to, consumer electronic products, parts of the consumerelectronic products, electronic test equipment, etc. Examples of theelectronic devices can also include, but are not limited to, memorychips, memory modules, circuits of optical networks or othercommunication networks, and disk driver circuits. The consumerelectronic products can include, but are not limited to, a mobile phone,a telephone, a television, a computer monitor, a computer, a hand-heldcomputer, a personal digital assistant (PDA), a microwave, arefrigerator, an automobile, a stereo system, a cassette recorder orplayer, a DVD player, a CD player, a VCR, an MP3 player, a radio, acamcorder, a camera, a digital camera, a portable memory chip, a washer,a dryer, a washer/dryer, a copier, a facsimile machine, a scanner, amulti functional peripheral device, a wrist watch, a clock, etc.Further, the electronic devices can include unfinished products.

CONCLUSION

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The word “coupled”, as generally usedherein, refers to two or more elements that may be either directlyconnected, or connected by way of one or more intermediate elements.Likewise, the word “connected”, as generally used herein, refers to twoor more elements that may be either directly connected, or connected byway of one or more intermediate elements. Additionally, the words“herein,” “above,” “below,” and words of similar import, when used inthis application, shall refer to this application as a whole and not toany particular portions of this application. Where the context permits,words in the above Detailed Description using the singular or pluralnumber may also include the plural or singular number respectively. Theword “or” in reference to a list of two or more items, that word coversall of the following interpretations of the word: any of the items inthe list, all of the items in the list, and any combination of the itemsin the list.

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “can,” “e.g.,” “for example,” “such as” and thelike, unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/orstates are included or are to be performed in any particular embodiment.

The above detailed description of embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example, whileprocesses or blocks are presented in a given order, alternativeembodiments may perform routines having steps, or employ systems havingblocks, in a different order, and some processes or blocks may bedeleted, moved, added, subdivided, combined, and/or modified. Each ofthese processes or blocks may be implemented in a variety of differentways. Also, while processes or blocks are at times shown as beingperformed in series, these processes or blocks may instead be performedin parallel, or may be performed at different times.

The teachings of the invention provided herein can be applied to othersystems, not necessarily the system described above. The elements andacts of the various embodiments described above can be combined toprovide further embodiments.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the disclosure. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the disclosure.

What is claimed is:
 1. A method of calibrating an envelope trackingsystem, the method comprising: generating a supply voltage for a poweramplifier using an envelope tracker having an envelope shaping tableincluding shaping data generated at a desired gain compression of thepower amplifier relating a plurality of scaled envelope signalamplitudes to a plurality of supply voltage levels; operating the supplyvoltage of the power amplifier at a first voltage level, the firstvoltage level associated with substantially no gain compression of thepower amplifier; measuring an output power of the power amplifier at thefirst voltage level; decreasing a voltage level of the supply voltageone or more times and measuring the output power at each decreasedvoltage level; determining a gain compression at each of the decreasedvoltage levels; determining a second voltage level of the supply voltageof the power amplifier corresponding to one of the decreased voltagelevels that is associated with the determined gain compression equal toabout that of the desired gain compression; retrieving the scaledenvelope signal amplitude associated with the second voltage level fromthe envelope shaping table; and calibrating the envelope tracker basedon the retrieved scaled envelope signal amplitude to correct for anenvelope amplitude misalignment of the envelope tracker.
 2. The methodof claim 1 further comprising scaling an amplitude of an envelope signalto generate a scaled envelope signal, the supply voltage generated atleast in part based on the scaled envelope signal.
 3. The method ofclaim 2 wherein the shaping data maps each of the plurality of scaledenvelope signal amplitudes to a corresponding one of the plurality ofsupply voltage levels.
 4. The method of claim 3 further comprisinggenerating the supply voltage from a battery voltage using the shapingdata and the scaled envelope signal.
 5. The method of claim 4 whereinthe shaping data is in a digital format, the method further comprisingconverting the shaping data into an analog format.
 6. The method ofclaim 2 wherein decreasing the voltage level of the supply voltageincludes changing calibration data of the envelope tracker to reduce thesupply voltage.
 7. The method of claim 6 wherein calibrating theenvelope tracker includes selecting a value of the calibration dataequal to about a value of the calibration data corresponding to thesecond voltage level.
 8. The method of claim 6 wherein scaling theamplitude of the envelope signal includes multiplying the envelopesignal by a scaling factor determined at least in part by thecalibration data.
 9. The method of claim 8 wherein the scaling factor isfurther determined by a power control signal from a transceiver.
 10. Themethod of claim 1 wherein measuring the output power of the poweramplifier at the first voltage level includes measuring the output powerusing a directional coupler and a power detector electrically coupled tothe output of the power amplifier.
 11. The method of claim 1 wherein thefirst voltage level is equal to about a maximum supply voltage of thepower amplifier.
 12. The method of claim 1 wherein decreasing thevoltage level of the supply voltage one or more times includesdecreasing the voltage level in discrete steps.
 13. The method of claim1 wherein decreasing the voltage level of the supply voltage one or moretimes and measuring the output power at each decreased voltage levelincludes decreasing the voltage level continuously and measuring theoutput power at a plurality of discrete voltage levels.
 14. Anon-transitory computer-readable storage medium comprising instructionsthat when executed by a processor perform a method of calibrating anenvelope tracking system, the method comprising: generating a supplyvoltage for a power amplifier using an envelope tracker, the envelopetracker having an envelope shaping table including shaping datagenerated at a desired gain compression of the power amplifier relatinga plurality of scaled envelope signal amplitudes to a plurality ofsupply voltage levels; operating the supply voltage of the poweramplifier at a first voltage level, the first voltage level associatedwith substantially no gain compression of the power amplifier; measuringan output power of the power amplifier at the first voltage level;decreasing a voltage level of the supply voltage one or more times andmeasuring the output power at each voltage level; determining a gaincompression at each of the decreased voltage levels; determining asecond voltage level of the supply voltage of the power amplifiercorresponding to one of the decreased voltage levels that is associatedwith the determined gain compression equal to about that of the desiredgain compression; retrieving the scaled envelope signal amplitudeassociated with the second voltage level from the envelope shapingtable; and calibrating the envelope tracker based on the retrievedscaled envelope signal amplitude to correct for an envelope amplitudemisalignment of the envelope tracker.
 15. A power amplifier systemcomprising: a power amplifier; an envelope tracker configured togenerate a supply voltage for the power amplifier, the envelope trackerincluding a shaping module having an envelope shaping table includingshaping data generated at a desired gain compression of the poweramplifier relating a plurality of scaled envelope signal amplitudes to aplurality of supply voltage levels, the envelope tracker furtherincluding a scaling module configured to scale an amplitude of anenvelope signal and to provide the scaled envelope signal amplitude tothe shaping module; a directional coupler electrically connected to anoutput of the power amplifier; a power detector electrically connectedto the directional coupler and configured to measure the output power ofthe power amplifier using the directional coupler; and a calibrationmodule configured to provide calibration data to the scaling module soas to change the scaled envelope signal amplitude generated by thescaling module, the calibration module configured to set the calibrationdata to a first value corresponding to a voltage level of the supplyvoltage associated with substantially no gain compression, and to reducethe voltage level of the supply voltage by changing the calibration datauntil the power detector indicates that the gain compression of thepower amplifier is equal to about the desired gain compression toprovide a reduced voltage level, the scaled envelope signal amplitudeassociated with the reduced voltage level in the envelope shaping tableis used to calibrate the envelope tracker to correct for an envelopeamplitude misalignment of the envelope tracker.
 16. The power amplifiersystem of claim 15 wherein the shaping data maps each of the pluralityof scaled envelope signal amplitudes to a corresponding one of theplurality of supply voltage levels.
 17. The power amplifier system ofclaim 16 further comprising a modulator configured to generate thesupply voltage from a battery voltage using the shaping data.
 18. Thepower amplifier system of claim 17 further comprising adigital-to-analog converter for converting the shaping data to analogdata for use by the modulator.
 19. The power amplifier system of claim15 further comprising a power control module electrically connected tothe power detector.
 20. The power amplifier system of claim 19 whereinthe scaling module is further configured to receive a power controlsignal from the power control module, the scaling module configured tochange the scaled envelope signal amplitude using the power controlsignal.
 21. The power amplifier system of claim 20 wherein the scalingmodule is configured to multiply the calibration data by the powercontrol signal to generate a scaling factor, and to multiply theamplitude of the envelope signal by the scaling factor to generate thescaled envelope signal amplitude.
 22. The power amplifier system ofclaim 15 wherein the first value of the calibration data corresponds toabout a maximum supply voltage of the power amplifier.
 23. The poweramplifier system of claim 15 further comprising a duplexer having aninput electrically connected to an output of the power amplifier and tothe directional coupler and an output electrically connected to anantenna.